Ovulation Calculator
Estimate your ovulation day and most fertile days from the first day of your last period and your average cycle length. See your fertile window, ovulation date, and next period for the next three cycles — worked out instantly in your browser.
Enter the first day of your last period to estimate ovulation.
Estimated ovulation day
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| Fertile window | Ovulation | Next period |
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This is an estimate for general information and planning only and is not medical advice or a method of contraception. Your input stays in your browser — nothing is sent anywhere.
How the ovulation calculator works
Ovulation — when an ovary releases an egg — usually happens about 14 days before your next period starts. The stretch from ovulation to the next period, called the luteal phase, is fairly constant at around 14 days for most people, while the first half of the cycle is what varies. So this calculator takes the first day of your last period and adds your cycle length minus 14 days to estimate your ovulation day.
For a typical 28-day cycle that places ovulation around day 14. If your cycles run longer, you ovulate later; if they run shorter, earlier. Enter your average cycle length — measured from the first day of one period to the first day of the next — to tune the estimate.
Around the ovulation day the tool marks your fertile window: the five days before ovulation plus the ovulation day itself. Sperm can survive in the body for up to about five days and the egg lives for roughly 24 hours, so these are the days when intercourse is most likely to lead to pregnancy. The table shows the window, ovulation day, and next period for the next three cycles so you can plan ahead.
Everything runs in your browser — the dates you enter are never uploaded or stored.
Common questions
How is ovulation calculated? By adding your cycle length minus 14 days to the first day of your last period, since ovulation typically occurs about two weeks before the next period.
What is the fertile window? The six or so days ending on ovulation day — the five days before it (because sperm survive up to five days) plus ovulation day itself (the egg lives about 24 hours).
How accurate is it? It is a calendar estimate that assumes a fairly regular cycle. Actual ovulation varies cycle to cycle, so use it as a guide and consider tracking basal body temperature, cervical mucus, or ovulation test kits for a closer reading.
Can I use it for birth control? No. Calendar estimates are not reliable contraception because they cannot account for cycle variation. Talk to your doctor about effective birth control.
What if my cycle length changes month to month? Use your average length. The more your cycles vary, the less precise the predicted day, so treat the fertile window as a range rather than an exact date.
This calculator is for general information and planning only. It is not a substitute for professional medical advice, diagnosis, or care.